Replication competent vaccinia virus (W), the current vaccine for smallpox, can cause severe complications after vaccination, especially in immune suppressed individuals. We are seeking to develop strains of VV that are replication competent, and thus will induce a strong immune response, without the complications associated with vaccination with the current vaccines. Thus, the Aim of this proposal is to prepare conditional mutants of VV that are either dependent on an FDA-approved drug for replication, or are treatable by an FDA-approved drug. For drug-sensitive viruses, individuals who experience complications could be treated with the FDA-approved drug. For drug-dependent viruses, should complications arise, the drug could be withdrawn from vaccinated individuals as a treatment. Drug-dependent viruses have the added advantage that they would not be able to spread in a viable form from vaccinated individuals to contacts. The relative immunogenicity and safety of these two strategies will be compared with that of a current vaccine (Dryvax) in immunocompetent mice (immunogenicity and safety), and in immunodeficient SCID mice (safety only). The most promising of these strains will be engineered into a virus background suitable for use in humans, prepared under GMP conditions and tested in chimpanzees and humans for safety and immunogenicity, compared to Dryvax.